1. Field of the Invention
The invention relates to bullet traps, and more specifically to bullet traps employed in rifle muzzle launched projectiles, which contain therein a tube for sliding the projectile over the muzzle of the rifle to retain the muzzle gas pressure, thus providing a means of propulsion; and within the tube a soft, low strength metallic plug to decelerate and retain the soft metallic components of rifle bullets; a hard, high strength metallic anvil to stop and retain the hard, high strength metallic components of rifle bullets; a shock absorbing piston to reduce the peak launch acceleration due to bullet impact; and a cap attached to the soft metallic plug, containing therein a resilient gasket to seal the bullet trap against the escape of rifle bullet and bullet trap fragments, thus providing greater safety to the operator.
2. Description of the Prior Art
The concept of launching a projectile, such as a grenade or line throwing device, from the muzzle of a rifle has existed for many decades, and offers advantages in terms of range and accuracy over hand-thrown counterparts. Such muzzle launched projectile systems commonly employ one of three bullet trap - rifle cartridge combinations. The first and most simple of these systems employs a blank propellant cartridge, fired into the projectile tube. Blank propellant cartridges can be designed to provide significant muzzle pressures, sufficient to launch the projectile with adequate velocity to a useful range, without the need to incorporate an expensive and heavy bullet stopping device within the projectile tube.
This concept of providing the soldier with sufficient blank propellant cartridges to launch rifle grenades had been satisfactory for many years. However, the growing need of modern armies to streamline logistics and reduce the weight burden on infantrymen eventually resulted in blank propellant cartridges becoming largely unavailable. To satisfy the continuing need to launch rifle grenades and line throwing projectiles without the availability of blank cartridges, two classes of bullet traps were developed, which safely operate when standard rifle ammunition is fired into the projectile tube.
The first type of bullet trap employs a soft metallic plug, such as aluminum, which is capable of decelerating and absorbing the impact of standard soft-core rifle bullets. Soft-core rifle bullets typically employ a soft lead-antimony filler within a ductile copper alloy bullet jacket. These bullet components deform and expand upon impact with an aluminum bullet trap plug. The aluminum plug has sufficient strength and ductility to retain the bullet, without fracturing the launch tube, thus allowing the safe launching of the projectile from the rifle.
In recent years, steel bullet cores have begun to replace portions of the soft lead bullet filler found in standard rifle ammunition, with the intent to improve some aspect of terminal effectiveness. Sometimes these steel cores are hardened to provide armor piercing capability. When these types of steel-core bullets are fired into soft aluminum bullet trap plugs, the steel core traverses the plug intact, and can cause catastrophic failure of the rifle grenade, and serious injury, if not death to the operator.
To improve the safety of these types of bullet traps with respect to steel-core bullets, a hardened steel anvil is typically placed directly following the softer aluminum plug. In this manner, the steel core is still retained, and the grenade safely launched from the rifle. The hardened steel anvil is not designed to replace the soft aluminum plug, however. The anvil serves to complement its effectiveness. If a steel anvil or steel plug was used alone, the impacting bullet would splatter, sending shards of bullet fragments in all directions, piercing the launch tube, with catastrophic results.
In addition to the use of steel anvils in bullet traps, interest developed in reducing the shock acceleration loads to which the projectile was subjected during launch. High launch acceleration loads can result in deformation and damage to the projectile payload. When a rifle bullet impacts a plug of aluminum, launch accelerations can exceed 30,000 times that of gravity. Many payload structures cannot withstand this level of acceleration loading. This bullet impact acceleration contrasts with the relatively minor peak acceleration generated from a blank propellant cartridge, approximately 5,000 g's.
To overcome these high bullet impact accelerations, bullet traps began to incorporate shock absorbing pistons. This piston can be a simple cylindrical section of aluminum, supported within the launch tube following the bullet trap, and which will crush and deform when the acceleration forces exceed the rupture strength of the material cross-section. By designing a relatively long and thin aluminum piston, peak accelerations may be reduced to under 10,000 g's.
Unfortunately, as a result of adding these steel anvils and shock absorbing pistons, bullet traps become heavier and longer. This added weight and depth to the bullet trap can greatly affect projectile performance. A longer bullet trap reduces the internal length of the projectile tube, which can be placed over the rifle muzzle, thus reducing the launch velocity. A lower launch velocity then results in less flight range of the projectile. The following explanation describes these tradeoffs.
When a bullet is fired into the projectile tube, the tube pressurizes with the cartridge gases from the rifle. These gases then begin to push the projectile off of the rifle muzzle. This entrapped gas pressure is the primary means of propulsion. Analysis and testing shows that the momentum of the bullet adds less than 12% to the launch velocity of the projectile. The greater the length of tube over the muzzle, therefore, the longer the pressure acts to accelerate the projectile. Therefore, to achieve equivalent launch velocity, with a longer bullet trap, the overall length of the projectile tube must be increased. This increase in tube length again increases the total projectile weight. Along with the added weight of the anvil, the total weight of the projectile can increase significantly. As the projectile total weight increases, the launch velocity decreases proportionally, since there is a fixed quantity of momentum in the rifle cartridge. As a result, overall system performance is again reduced.
To limit this cycle of increased weight and reduced launch velocity, due to the need to retain steel-core bullets within the projectile launch tube, a third class of bullet trap was developed. This type is called the bullet-through, and is exemplified in U.S. Pat. No. 4,394,836 (Chavee et al.). This type of bullet trap simply employs a resilient rubber gasket supported within the launch tube, which permits the bullet to traverse the entire length of the projectile, and exit from the front, while the cartridge gas pressure is retained within the tube behind the self-sealing gasket. The bullet-through approach, however, assumes that the payload can accommodate a through-hole up front, so that the bullet may exit.
In spite of the development of the bullet-through class of bullet traps, there exists a continuing need for actual bullet traps employing soft metallic plugs, followed by hardened metallic anvils. Some muzzle launched projectiles cannot be efficiently adapted to the bullet-through approach, such as the muzzle launched grapnel hook, U.S. Pat. No. 5,448,937 (Buc et al.). For these types of muzzle launched projectiles, which employ a solid payload mass forward of the bullet trap, a more conventional approach is still required to safely launch from a rifle with a standard soft-core and steel-core rifle bullet. In addition, the bullet trap configuration must be inexpensive to fabricate and assemble, and must maximize the internal tube length available to place over the rifle muzzle.
Recently, an additional operational requirement has developed, which further stresses conventional bullet trap design. It has been observed during extensive testing that occasionally a bullet fragment or bullet trap particle can escape rearward from the projectile launch tube, once the projectile has cleared the rifle muzzle. This is a common, largely unpredictable, and potentially hazardous condition, which is not easily or consistently solved by modifying the soft aluminum plug material or shape.
The solution to this problem is to incorporate a resilient rubber gasket supported within the launch tube before the soft aluminum plug. This gasket, if properly affixed within the tube, is pierced by the bullet, yet re-seals, retaining all bullet and bullet trap fragments down stream in the bullet trap. Such an approach has been proposed in several U.S. Pat. Nos. in particular 3,664,263 (Driscoll), 3,726,036 (Jennings et al.), and 5,349,906 (Devaux et al.). However, the level of complexity and questionable performance of these inventions make them of little practical use for many muzzle launched projectile applications, such as within the muzzle launched grapnel hook, and with a variety of rifle bullets employing both soft lead fillers and hardened steel core components. Some of these bullet traps employ many complicated to manufacture subcomponents, within a complex machined projectile tube, requiring numerous boring, reaming, and threading operations, in order to accurately fit the bullet trap components within the projectile tube. Some teach that the bullet trap components are placed within the tube from the front of the projectile--the opposite end from which the bullet enters--which adds additional costs, since this separate tube must then be affixed to the payload.
It is also doubtful whether the rubber gasket these bullet traps will function as intended, once the gasket has been pierced by the bullet, and the bullet trap becomes pressurized. Understanding that a certain amount of gas pressure follows the bullet through the gasket into the bullet trap, at some point during projectile travel off of the rifle muzzle, the gas pressure within the bullet trap will greatly exceed that within the projectile tube. The bullet trap is also highly pressurized, since it is initially exposed to the high muzzle pressures, which can approach 10,000 psi. Due to adiabatic expansion, the pressure within the tube will drop dramatically with every inch of projectile travel, down to a low point of perhaps 500 psi when the projectile finally leaves the rifle muzzle. At this point the gasket will re-open, permitting pressurization to violently escape rearward out of the bullet trap, along with bullet and bullet trap fragments. Since prior designs do not provide an adequate means to resist this negative pressure against the gasket, the intent of the gasket to retain bullet fragments is not achieved.
In light of these shortcomings in the approaches proposed in the prior art, we have greatly improved upon the concept of sealing the bullet trap and have invented a much simplified, yet more reliable configuration, employing a fragment-sealing rubber gasket and only three inexpensive to machine bullet trap components. This self-contained bullet trap unit is then simply dropped into the back of the projectile launch tube, fabricated with one inexpensive boring operation, followed by one simple reaming operation. Our much simplified bullet trap approach provides results unanticipated by the prior art, within a simplified and inexpensive to manufacture configuration, providing heretofore unrealized advantages to-bullet trap and muzzle launched projectile performance.